The present invention relates to an imaging lens system with two lenses, especially to an imaging lens system for mobile phones or mini-cameras with image sensors such as CCD (Charge Coupled Device) or CMOS (Complementary Metal Oxide Semiconductor). The imaging lens system is formed by two lenses with features of long back focal length and low cost.
The electronics available now become more compact and multifunctional. Most of them such as digital cameras, PC (personal computer) cameras, network cameras and mobile phones, even personal digital assistants (PDA) are equipped with an image lens system. The imaging lens system not only requires good imaging quality but also needs compact volume and lower cost. Moreover, the imaging lens system with larger field angle can improve image quality of the electronics and match users' requirements.
There are various designs such as two lenses, three lenses, four lenses or five lenses of the imaging lens system applied to mini electronics. Yet while a compromise of resolution and cost, the two lenses is preferred.
There are various structures of conventional two lenses imaging lens system and the difference among them or technical character is in the shape of the two lenses, location of the convex surface/concave surface, positive/negative refractive power, or relative optical parameters. Among these designs, the combination of a first lens with positive refractive power and a second lens with positive refractive power can achieve requirement of minimized volume, such as prior arts revealed in US2005/0073753, US2004/0160680, U.S. Pat. No. 7,110,190, U.S. Pat. No. 7,088,528, US2004/0160680, EP1793252, EP1302801, JP2007-156031, JP2006-154517, JP2006-189586, TWM320680, TWI232325, and CN101046544 etc.
However, the volume of these imaging lens systems still requires further improvement. For larger field angle, the imaging lens system in US2008/0030875 includes a lens with positive refractive power and a lens with negative refractive power, the imaging lens system in U.S. Pat. No. 5,835,288 is formed by combinations of biconcave lenses and biconvex lenses, the systems in JP08-334684, JP2005-107368 use combinations of positive/positive refractive power or negative/positive refractive power. Or as shown in JP2004-177976, EP1793252, EP1793254, U.S. Pat. No. 6,876,500, US2004/0160680, U.S. Pat. No. 7,088,528, TWI266074, the combination of the lens with positive refractive power and the lens with positive refractive power.
A common feature of the imaging lens systems described above is that the back focal length is short. The disadvantage of short back focal length is when the electronics become more compact, the imaging lens systems will become more compact with the electronics; therefore, the back focal length is getting shorter with the proportion of the imaging lens systems. Besides the lenses, the imaging lens system also includes a IR (infrared) cut-off filter and a glass. Because thickness of the IR cut-off filter and the glass is fixed, when the imaging lens system becomes more compact, the back focal length that gets shorter with the proportion of the imaging lens system is short enough to dispose the IR cut-off filter and the glass.
Therefore, the users require the imaging lens system with long back focal length. The present invention provides a better design of the imaging lens system applied to electronics such as mini cameras and camera phones.